The present invention relates to organic furan compounds and to the process for synthesizing them. The invention also relates to a process for crosslinking polymers and to the polymers thus obtained.
The novel organic furan compounds are particularly suitable as chain-extending agents in the process for crosslinking or branching polymers.
Organic furan compounds containing at least two furyl rings are already known. Compounds that are well known are furfuraldazine and bis(furfurylidene)acetone, which are used as agents for crosslinking polymers.
The abovementioned furfuryl derivatives, currently used as crosslinking agents, have a number of drawbacks. One drawback is the dark brown coloration acquired by polymers when they are crosslinked with the known furfuryl derivatives. In addition, the known furfuryl derivatives are not easy to manipulate. The reason for this is that a certain number of the said furfuryl derivatives are hazardous to human health, since the said derivatives have mutagenic properties. In addition, they have the property of soiling materials with which they come into contact. At high temperature, materials containing the soiling furfuryl derivatives can release the soiling due to the effect of its volatility, even if they are not in direct contact with other materials.
The abovementioned furan compounds also comprise the category of polyol esters of 3-(2-furyl)-2-propenoic acid. Known compounds belonging to this category are diol esters containing fluorine atoms in the alkylene chain, which are described in xe2x80x9cDie Angewandte Makromoleculare Chemie 199 (1992) 149-170xe2x80x9d. These diol esters are used for preparing partially fluorinated photopolymers.
Novel diol esters of 3-(2-furyl)-2-propenoic acid have now been found which do not have the abovementioned drawbacks when used as agents in the process for crosslinking or branching polymers.
In addition, the esters according to the present invention show high reactivity with polymers in radical form. Thus, they are capable of crosslinking the polymer even if they are used in very low amounts.
The present invention thus provides diol esters of 3-(2-furyl)-2-propenoic acid having the following general formula:
A2Xxe2x80x83xe2x80x83(I)
in which
A is 
and X is a C1-C6 alkylene group. The alkylene group may contain one or more substituents (in place of one or more hydrogen atoms) chosen from linear or branched C1-C20 alkyl radicals, C3-C18 cycloalkyl radicals, C6-C18 aryl radicals, for instance a phenyl group, or C7-C20 arylalkyl radicals. The cycloalkyl and aryl radicals may also contain one or more substituents such as a C1-C20 alkyl group.
Preferably X is an alkylene group containing two, three or four carbon atoms.
The diol esters of the present invention are free of halogens.
Specific examples are the following esters:
methanediyl bis[3-(2-furyl)-2-propenoate];
phenylmethanediyl bis[3-(2-furyl)-2-propenoate];
1,2-ethanediyl bis[3-(2-furyl)-2-propenoate]; 
1,3-propanediyl bis[3-(2-furyl)-2-propenoate]: 
1,2-propanediyl bis[3-(2-furyl)-2-propenoate];
1,3-butanediyl bis[3-(2-furyl)-2-propenoate];
1,4-butanediyl bis[3-(2-furyl)-2-propenoate];
2,3-butanediyl bis[3-(2-furyl)-2-propenoate];
1,5-pentanediyl bis[3-(2-furyl)-2-propenoate];
1,6-hexanediyl bis[3-(2-furyl)-2-propenoate];
4-methyl-2,4-pentanediyl bis[3-(2-furyl)-2-propenoate].
The novel esters may be synthesized according to various processes. For example, they may be sythesized by known esterification reactions, by reacting the corresponding saturated aliphatic alcohols containing two hydroxyl groups (diols) with 3-(2-furyl)-2-propenoic acid or derivatives thereof, for instance acyl halides and anhydrides.
Both the said alcohols and the 3-(2-furyl)-2-propenoic acid are well-known compounds. Examples of diols are 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 4-methyl-2,4-pentanediol and 1,5-pentanediol.
The methanediyl bis[3-(2-furyl)-2-propenoate] may be synthesized by reacting 3-(2-furyl)-2-propenoic acid or the corresponding anhydride with formaldehyde in the presence of an acid catalyst, such as p-toluenesulphonic acid. The methanediyl bis[3-(2-furyl)-2-propenoate] is then separated from the by-products, for example by chromatography.
The phenylmethanediyl bis[3-(2-furyl)-2-propenoate] may be synthesized by reacting 3-(2-furyl) -2-propenoic anhydride with toluene in the presence of CrO3.
The present invention also provides a crosslinkable polymer composition which comprises a crosslinkable polymer, an ester of formula (I) and a free-radical initiator, and a process for crosslinking polymers in the presence of a free-radical initiator and an ester of formula (I). Any crosslinkable polymer, either homopolymers or copolymers, and blends thereof, may be used in the present invention. However, polyolefins are preferred.
Suitable examples are crystalline and partially crystalline polymers of xcex1-olefins CH2xe2x95x90CHR, in which R is a hydrogen radical or a C1-C8 alkyl radical. Polymers that are particularly preferred are ethylene polymers, for instance HDPE, LDPE and LLDPE, propylene polymers, especially isotactic or mainly isotactic polymers, and crystalline copolymers of propylene with ethylene and/or C4-C10 xcex1-olefins, such as, for example, 1-butene, 1-hexene, 4-methyl-1-pentene or 1-octene, in which the total content of comonomer ranges from 0.05% to 20% by weight relative to the weight of the copolymer, or blends of the said copolymers with isotactic or mainly isotactic propylene homopolymers.
Other suitable polyolefins are saturated and unsaturated elastomers, such as ethylene-xcex1-olefin rubbers, that is to say copolymers of ethylene with one or more types of C3-C10 xcex1-olefin and, optionally, a diene. Examples of xcex1-olefins include propylene, 1-butene, 1-pentene, 1-hexene and 1-octene. The ethylene content is generally up to about 75% by weight. Examples of dienes include 1,4-hexadiene, 1,5-hexadiene, ethylidene-1-norbornene and dicyclopentadiene. The dienes are generally in a content of from 1% to 10% by weight. Other examples of the said rubbers are copolymers of ethylene and vinyl acetate and derivatives thereof, copolymers of ethylene and acrylic acid esters or derivatives thereof, copolymers of ethylene and methacrylic acid or derivatives thereof, or the like. Further examples of unsaturated elastomers are butadiene rubbers, styrene-butadiene rubbers (SBR), norbornadiene rubber (NR), and the like.
Blends of the abovementioned polyolefins are also possible. Examples of such blends are heterophasic copolymers comprising (I) a propylene homopolymer and/or one of the said crystalline propylene copolymers and (II) an elastomeric fraction comprising one or more of the said elastomeric copolymers, typically prepared according to known processes by mixing the components in the melt, or by sequential polymerization, and generally containing the elastomeric fraction (II) in an amount of from 5% to 80% by weight.
The esters of formula (I) as crosslinking agents are generally used in an amount of from 0.01 to 8 parts by weight, preferably from 0.15 to 5 parts by weight and more preferably from 0.15 to 2 parts by weight, per 100 parts of crosslinkable polymer.
The amount of free-radical initiator in the crosslinking process generally ranges from 0.1% to 10% by weight, preferably from 0.2% to 5% by weight and more preferably from 0.2% to 2% by weight, relative to the crosslinkable polymer.
Suitable free-radical initiators have a decomposition half-life of 10-200 seconds in the temperature range in which crosslinking reaction is normally carried out (generally from 100xc2x0 C. to 240xc2x0 C. for polyolefins).
Examples of free-radical initiators are organic peroxides, for instance diacyl peroxides, dialkyl peroxides, peroxyesters and perketals. Specific examples thereof are benzoyl peroxide, dicumyle peroxide, di-t-butylperoxydiisopropylbenzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane.
An indirect evaluation of the degree of crosslinking may be provided by the amount of gel which is formed by the crosslinking effect, the gel reducing the solubility of the polymer. Such amount is calculated using the following formula:
% of gel=(Cxe2x80x94X)xc3x97(1/C)xc3x97100
in which C is the percentage of the crosslinked polymer in the composition before crosslinking, while X is the soluble fraction of the crosslinked polymer.
This crosslinking process is not specifically limited and may be any conventional process known to those skilled in the art. One example is the dynamic crosslinking process. The crosslinking is normally carried out from 140 to 240xc2x0 C. for a period of from 1 to 60 minutes. Preferably, the process is carried out under an inert atmosphere, for example nitrogen.
Common additives that are usually used with polymers, in particular polyolefins, for instance heat stabilizers, antioxidants, pigment charges, mineral fillers, flame retardants, antistatic agents and lubricants, may be added to the polymer before, during or after the crosslinking stage.
As stated above, the crosslinked polymers according to the invention show less yellowing than polymers crosslinked with the known furfuryl derivatives, for instance furfuraldazine. A marked decrease in coloration is observed in particular when the esters are used in small amounts, that is to say up to about 0.4 part by weight per 100 parts of the crosslinkable polymer. The following examples are given to illustrate the present invention without, however, constituting a limitation thereof.
The processes used to obtain the property data given in the examples and in the description are described below.
The melting point is measured using a Reichert(copyright) Polyvar optical microscope with cross-polarizers, equipped with a Mettler(copyright) FP52 programmable heater, and the temperature ramp used is 10xc2x0 C. per minute.
The volatilization temperature is measured by thermogravimetric analysis carried out using a Mettler(copyright) TG50 instrument, and the temperature ramp used is 20xc2x0 C. per minute.
The xylene-insoluble fraction is determined as follows. A weighed amount of the sample (2 grams) is suspended in 200 mL of anhydrous xylene at room temperature; the suspension is heated to the boiling point of the solvent (135xc2x0 C.) with continuous stirring, and the solution is maintained at reflux for one hour. The hot solution is then filtered to remove the soluble fraction, and the residue is washed with 50 ml of boiling xylene, until the filtrate no longer shows any precipitation of polymer. The residue is then washed with cold acetone and dried to constant weight.
The melt flow index L (MIL) is measured according to ASTM process D 1238, condition L.
The colour is estimated by eye.